Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and compared to spark-ignited stoichiometric gasoline engines, have significantly lower emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Emissions of diesel engines include particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
There are major differences between catalyst systems used to treat diesel engine exhaust gas and gasoline engine exhaust gas. A significant difference between the two types of engines and their operation is that gasoline engines are spark ignited and operate within a stoichiometric air to fuel ratio, and diesel engines are compression ignition engines that operate with a large excess of air. The emissions from these two types of engines are very different and require completely different catalyst strategies. Generally, the treatment of diesel emissions is more complicated than gasoline engine emissions treatment due to the formation of high amounts of NOx and particulate matter in diesel engines.
The two major components of exhaust particulate matter are the soluble organic fraction (SOF) and the soot fraction (soot). The SOF condenses on the soot in layers, and is derived from unburned diesel fuel and lubricating oils. The SOF can exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the exhaust gas. Soot is predominately composed of particles of carbon. The particulate matter from diesel exhaust is highly respirable due to its fine particle size, which poses health risks at higher exposure levels. Moreover, the SOF contains polycyclic aromatic hydrocarbons, some of which are suspected carcinogens.
Diesel engines utilizing new advanced combustion technologies such as Homogeneous Charge Compression Ignition (HCCI) will be able to reduce engine out NOx and particulate matter (PM) emissions by reducing the combustion flame temperature within the engine cylinder and by increasing the uniformity and mixing of the fuel charge prior to ignition. Generally, the emitted exhaust gas prior to any treatment contains significantly reduced particulate matter and NOx as compared to the exhaust gas emitted from traditional diesel engines. In some instances, the NOx emissions from such advanced combustion diesel engines is two to three times lower than the emissions from traditional diesel engines. However, in the process of changing the combustion process to lower NOx and PM emissions, the overall quantity of CO and hydrocarbon (HC) emissions will increase, the nature of the HCs formed will change (e.g. more methane may be produced), and the exhaust temperature may be lowered. In some instances, the CO and HC emissions from advanced combustion diesel engines is 50% to about 100% higher than the HC and CO emissions from traditional diesel engines. Since these exhaust characteristics will create significant challenges for current diesel emission catalyst technology, new catalyst formulations are needed in order to meet increasingly stringent environmental regulations such as Euro 6 and US Tier 2 Bin 5.
Oxidation catalysts comprising a precious metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both hydrocarbon and carbon monoxide gaseous pollutants by catalyzing the oxidation of these pollutants to carbon dioxide and water. Such catalysts have been generally contained in units called diesel oxidation catalysts (DOC), or more simply catalytic converters or catalyzers, which are placed in the exhaust flow path from diesel power systems to treat the exhaust before it vents to the atmosphere. Typically, the diesel oxidation catalysts are formed on ceramic or metallic substrate carriers (such as the flow-through monolith carrier, as described herein below) upon which one or more catalyst coating compositions are deposited. In addition to the conversions of gaseous HC and CO emissions and particulate matter (SOF portion), oxidation catalysts that contain platinum group metals (which are typically dispersed on a refractory oxide support) promote the oxidation of nitric oxide (NO) to NO2.
Catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating exhaust gas emissions from diesel engines. Platinum (Pt) is an effective metal for oxidizing CO and HC in a DOC after high temperature aging under lean conditions and in the presence of fuel sulfur. On the other hand, Pd-rich DOC catalysts typically show higher light-off temperatures for oxidation of CO and HC, especially when used to treat exhaust containing high levels of sulfur (from high sulfur containing fuels) or when used with HC storage materials. “Light-off” temperature for a specific component is the temperature at which 50% of that component reacts. Pd-containing DOCs may poison the activity of Pt to convert hydrocarbons and/or oxidize NOx and may also make the catalyst more susceptible to sulfur poisoning. These characteristics have typically prevented the use of Pd-rich oxidation catalysts in lean burn operations, especially for light duty diesel applications where engine temperatures remain below 250° C. for most driving conditions.
Although platinum (Pt) has good light-off characteristics for CO and HC and, therefore, has historically been the preferred precious metal of choice for catalyst compositions used to abate diesel engine exhaust, palladium (Pd) recently has become of greater interest due to its relatively lower cost. In some cases, palladium has proven to be suitable in DOC catalysts in conjunction with platinum to reduce the required amount of platinum, despite it being more sensitive to sulfur and somewhat less reactive on a weight basis. In fact, the combination of Pt and Pd may be more active than Pt alone. Due to the lower reactivity of palladium in DOC catalysts, it is important to ensure that it is located in the DOC catalyst in a way that does not inhibit its performance.
Oxygen storage components such as cerium are not typically mixed with DOCs because the combination would result in the platinum remaining in the oxidized state. Since normal diesel engines operate under constantly lean conditions, the platinum would have no opportunity to be reduced to the active metallic form.
As emissions regulations become more stringent, there is a continuing goal to develop diesel oxidation catalyst systems that provide improved performance, for example, lower light-off temperature. There is also a goal to utilize components of DOCs, for example, palladium, as efficiently as possible.